extent, variation in muscle hydration influences lean mass estimates by DXA. There appears to be no influence of age on DXA physical models used for estimating the three main components as shown in Table 12-6. Pietrobelli et al. (1998) observed similar attenuation ratios (i.e., r values) for skeletal muscle based on chemical analysis of biopsies in subject groups ranging in age from newborns to adults. Hence, for a low cost and with minimal radiation exposure, DXA allows quantification of a large proportion of total body muscle mass.

Skeletal muscle tissue has a relatively small extracellular fluid compartment and large myofibrillar mass (Synder et al., 1975). As a result, skeletal muscle is relatively rich in intracellular potassium when referenced to other components such as total body protein (i.e., nitrogen). Anderson (1963) and later Burkinshaw (1978) and their colleagues exploited this property of muscle tissue to develop multicomponent models. Notably, Burkinshaw's classic model was based on a known and constant ratio of potassium (K) to nitrogen (N) in muscle (3.03 mmol/g) and non-muscle lean tissue (1.33 mmol/g) (Figure 12-14). Burkinshaw, and later Cohn et al. (1980), developed their models based on chemically analyzed tissue samples. A two-component model can be developed:

Where TBK is total body potassium and TBN is total body nitrogen, prompt-γ neutron activation analysis is used to measure TBN, and whole-body ⁴⁰K counting is used to measure TBK (Cohn et al., 1980).

Although the K:N model is of historic interest, there are a number of problems now recognized with the method. First, the K:N ratio is probably not constant in muscle and non muscle lean tissues but may change with age and other factors such as physical activity level. As a result, large between-individual differences most likely exist in the above model constants. Accordingly, Wang et al. (1996) observed a significant correlation between total